The present invention relates to electric assisted steering systems (EAS) in motor driven road vehicles and is concerned in particular with a control system in a road vehicle adapted to provide steering torque compensation or haptic torque based on the measured vehicle dynamics, such as yaw rate or lateral acceleration.
Electric assist steering systems are well known in the art. Electric assist steering systems that use, for example, a rack and pinion gear set to couple the steering column to the steered axle, provide power assist by using an electric motor to either apply rotary force to a steering shaft connected to a pinion gear, or apply linear force to a steering member having the rack teeth thereon. The electric motor in such systems is typically controlled in response to (a) a driver's applied torque to the vehicle steering wheel, and (b) sensed vehicle speed.
Other known electric assist steering systems include electro-hydraulic systems in which the power assist is provided by hydraulic means under at least partial control of an electrical or electronic control system.
There is a desire, at least in certain vehicle market segments, to provide the driver with information about the dynamic state of the vehicle via the steering wheel torque. The effects that are most common are an increase in driver torque as the lateral acceleration on the vehicle increases (the handwheel seems to become heavier), and a sudden drop in driver torque (the handwheel seems to become much lighter) when the vehicle reaches terminal understeer. (Terminal understeer is considered to be when an increase in handwheel angle, no longer gives an increase in vehicle yaw rate.) Traditionally these effects have been produced by careful design of the steering system, but modern power assisted steering systems, and space and other compromises in the design of steering systems, has lead to the effects becoming much less noticeable. However, there is a general perception that these effects improve the handling of a vehicle, and therefore they can be quite important in certain market segments.